1. Field of the Invention
The present invention relates to a method of and a circuit arrangement for processing color video signals, particularly NTSC color television signals.
2. Brief Description of the Prior Art
In the NTSC standard, the phase of the chroma signal is reversed on successive horizontal lines and, thus, inverted. This condition lends itself to comb filtering to separate the luminance components from the chrominance components. For the sake of simplicity, luminance will hereinafter be referred to as "luma" and chrominance as "chroma". In comb filtering, successive horizontal lines of the video signal are added and subtracted together. The addition provides the luma signal component (the chroma components are oppositely phased and are thus cancelled in the addition), and the subtraction provides the chroma signal component (the luma components are cancelled in the subtraction because thev are always in phase). Comb filtering produces high-quality separation of the chroma and luma signals except where there are significant changes in the luma or chroma signal from line to line. In those cases, comb filtering introduces distortions and artifacts because of the dissimilar information that is being combined.
In color signal processing without comb filtering, hereinafter referred to as "simple decoding", the luma-chroma compbsite signal is passed through a selective color sub-carrier frequency trap (=color trap) to obtain the luma component, and through a chroma bandpass filter to cbtain the chroma component. The 3.58-MHz color trap destroys luma resolution in that frequency range. The band-pass filter, which has a mid-frequency of 3.58 MHz, also passes luma components which mix with the chroma components, thus creating untrue colors. The benefit of comb filtering is that luma resolution is not reduced in the vicinity of the color subcarrier frequency, and that color decoding is not disturbed by luma components. On the pther hand, comb filtering of video signals which have significant vertical transitions, particularly color transitions, produce various artifacts that can be very disconcerting to a viewer.
A certain remedy for these disturbing effects is provided by the well-known "two-line comb filter", which combines the video signals from three horizontal lines, e.g., the line of concern and one line above and one line below. In that two-line comb filter, luma/chroma separation by comb filtering is effected only when there are no vertical transitions present. If vertical transitions exceed a predetermined level, e.g., a noise threshold, no signal combing is performed. The prior art circuit samples vertically aligned pixels from the line above and the line below, and compares the sample values with a reference value formed from the sample values of horizontally aligned pixels. It then makes a decision as to whether to "two-line comb" (upper and lower horizontal lines with the center horizontal line) or to simple decode the video signal. The sampled video display area includes seven video pixels; five adjacent pixels on the horizontal line being processed, one pixel on the line above, and one pixel on the line below. The comparison is performed essentially only with the chroma component of the pixels, because the video signal was previously filtered by means of a chroma bandpass filter.
The decision to either two-line comb process or to simple decode is based upon comparing the differences between the two vertically aligned pixels in the upper and lower lines and the difference between the end pixels of the horizontally aligned pixels (=line). The prior art circuit thus covers only the two main directions in the vicinity of the pixel to be processed. The system lacks, for example, any diagonal comparisons of pixels and is consequently limited in its ability to determine when comb processing is desirable. Consequently, the prior art system often introduces undesirable artifacts into the video display since proper combing operation does not occur at many critical situations where the line at the center is different from the two upper and lower lines. Further, the prior art circuit is highly noise-sensitive since the noise threshold is fixed and not adaptive to the incoming video signal.